A thermoplastic fluororesin such as a tetrafluoroethylene-hexafluoropropylene copolymer or an ethylene-tetrafluoroethylene copolymer, is excellent in thermal resistance, chemical resistance, weather resistance, etc., and thus is widely used as a material for tubes, pipes, coatings, wire-coverings, films, horticultural covering films, etc.
However, the thermoplastic fluororesin has had a problem such that during the blow molding, when a cylindrically shaped resin (hereinafter referred to as a parison) in a softened state in the mold is pulled downward by its own weight, an upper portion of the parison stretches so that the wall thickness of that portion becomes thin, and the wall thickness of the molded product becomes non-uniform. Such reduction of the wall thickness becomes distinct especially in blow molding a large sized product. Further, in inflation molding, the thickness of a film to be formed was likely to be non-uniform.
In order to prevent non-uniformity in the wall thickness of a parison or to make the thickness of the film to be uniform, it is effective to increase the melt tension of the thermoplastic fluororesin. If the molecular weight of the thermoplastic fluororesin is increased, the melt tension can be made high. However, the melt viscosity of the thermoplastic fluororesin at the time of molding becomes high, whereby there has been a problem such that the moldability of the thermoplastic fluororesin at the time of extruding a parison tends to be low, and the productivity tends to be low.
A polyethylene is known to have a high melt tension and a low melt viscosity when it has a long chain branched structure. Patent Document 1 reports that a copolymer of tetrafluoroethylene with a fluorinated monomer having at least two double bonds, is excellent in blow moldability, but discloses nothing about an ethylene-tetrafluoroethylene copolymer.    Patent Document 1: JP-A-2002-12626